Some active medical implants are powered by primary battery cells. Depending on their power consumption, they need to be replaced by new implants with new primary cells. The required surgery causes stress for the patients, various typical risks (e.g. infections, anaesthetic, etc.), and costs (new implant, operation costs, etc.).
Using rechargeable batteries in a medical implant can minimize the number of re-implantations and thereby reduce patient risk and cost. An implant with a rechargeable battery that provides tens of mAh of electric power (e.g., a rechargeable LiPo-battery) should have a recharge period that provides for patient comfort and acceptance. Typically that may be achieved by a charging power of 100 mW or higher.
Percutaneous implant connections can create surgical and medical risks such as risk of infections and biofilms by percutaneous connectors. Thus a wireless charging arrangement is usually preferable. This can be achieved using inductive links such as in the Precision Plus™ SCS System from Boston Scientific and the RestoreSensor/RestoreAdvanced/RestoreUltra SCS by Medtronic. Such inductive link arrangements generally use an external charger device on the patient's skin outside the body, which includes an external charging coil and necessary electronics. And there also is a corresponding internal coil being part of the implant. The coils are magnetically coupled by the inductive link with a power transfer an alternating magnetic field produced by the external coil, where some fraction k (coupling factor) of that field penetrates the skin to the implant coil which induces a voltage that drives current for the implant electronics.
For a given distance between transcutaneous induction coils, the magnetic coupling is primarily a function of the coil shapes, which typically are circular or spiral. Ideally the best coupling factor can be achieved if both coils have the same diameter, which should be at least √2 times the distance between the coils. See e.g., K. Finkenzeller, RFID Handbook Fundamentals and Applications in Contactless Smart Cards and Identification, (3rd ed.), Wiley, Hoboken, N.J. (2010), incorporated herein by reference. For example, given a coil distance 20 mm, optimal coupling requires a coil radius of 28 mm, which is too large for most implants.
For the patient's convenience, the external charging device should be compact and portable with its own integrated battery. To keep such an external device small and to minimize self-heating of adjacent tissues from eddy currents, the energy transfer needs to be efficient. Eddy current induced self-heating is not just uncomfortable, but temperature upper limits are also set by laws and regulations.
The Medtronic charging device is a quite bulky system running at a frequency of 175 kHz that reportedly suffers from significant self-heating issues. A similar problem is reported by St. Jude Medical. U.S. Pat. No. 7,599,744 (incorporated herein by reference) describes a transcutaneous coil arrangement that uses a rod made of soft iron which redirects the magnetic field lines in a defined way.